1. Field of the Invention
This invention relates to a novel metal ion-exchanged phosphorus-vanadium oxide and to a solid acid catalyst which contains this phosphorus-vanadium oxide. More particularly, this novel phosphorus-vanadium oxide is a compound which exhibits a specific d value (lattice spacing, .ANG.). This compound manifests the qualities of a solid acid and, owing to the acid qualities, can be used as a catalyst.
2. Description of the Prior Art
Various studies have been heretofore made concerning the phosphorus-vanadium oxides and physical properties and uses of these oxides have been simultaneously explored for the sake of development. The phosphorus-vanadium compounds are specific in such qualities as the state of oxidation and the configuration and consequently have various crystal structures and find various applications owing to these structures. It is well known that the catalytically effective component of phosphorus-vanadium oxides is divanadyl pyrophosphate, a crystalline oxide which has the composition of (VO).sub.2 P.sub.2 O.sub.7. This divanadyl pyrophosphate is obtained by synthesizing its precursor, vanadyl hydrogen orthophosphate hydrate (VOHPO.sub.4.nH.sub.2 O), and thermally dehydrating this precursor. It is also known that the vanadyl hydrogen orthophosphate hydrate as the precursor is an interlayer compound which is formed by the hydrogen bondage of opposed VOHPO.sub.4 layers through the medium of water molecules intervening therebetween.
It is widely known that phosphorus-vanadium compounds generally form an effective catalyst for gas-phase oxidation of such hydrocarbons as butane, butene, and butadiene which have four carbon atoms (hereinafter referred to briefly as "C4 hydrocarbons"). Various patterns of X-ray diffraction peaks for identifying these phosphorus-vanadium oxides have been published (JP-A-53-61,588, JP-A-56-41,816, JP-A-56-45,815, JP-A-59-132,938, and JP-A-05-15,781). Various inventions concerning the addition of third components to the phosphorus-vanadium oxides have been also disclosed (JP-A-52-135,580, JP-A-54-30,114, and JP-A-57-111,219).
Reports on phosphorus-vanadium oxides are published in various articles of literature besides the patent publications mentioned above. B. K. Hodnett. ed., Catalysts Today, Vol. 1, No. 5 (1987), for example, carries a detail report.
Datta et al Chem. commun, (1996) pages 89-91 disclose palladium added phosphorus-vanadium compounds prepared by two methods.
The compound prepared form an organic solvent solution shows the same XRD pattern as that of the original vanadyl hydrogen orthophosphate (VOHPO.sub.4 .0.5H.sub.2 O) even if palladium is added, so palladium is not incorporated into the structure thereof, then it is a state of a mixture.
There is possibility that the compound prepared by another method using an aqueous medium is similar compound to a compound wherein a proton between interlayers disclosed in the present invention is exchanged with a divalent metal ion as being clear from XRD pattern of FIG. 6. Especially, about the peak of 7.04 .ANG. in the XRD pattern of FIG. 6, it is thought that there is possibility overlaps with the metal ion-exchanged compound in accordance with the present invention. Then preparation of a palladium added compound was carried out by a method of Datta et al, and comparison between the compound obtained by the method of the present invention and the compound obtained by the method of Datta et al.
Thus, a compound having the XRD pattern of FIG. 1-2a cannot be prepared by the method of Datta et al. About vanadyl hydrogen orthophosphate (VOHPO.sub.4.0.5H.sub.2 O) which is a precursor, similar compounds can be obtained by both methods using an aqueous mediumoranorganic medium. However, about the palladium added compound, a similar compound shown in FIG. 1-2a cannot be obtained. Further, when Pd ion was measured, but it can almost be detected, so Pd is not exchanged with the proton between interlayers.
Then we have compared the method between the present invention and Datta et al to obtain the difference thereof. In the method of Datta et al, the experiments were carried out using vanadyl hydrogen orthophosphate (VOHPO.sub.4.0.5H.sub.2 O) in both organic medium and aqueous medium, while in the present invention, the experiments were carried out by ion-exchange method using vanadyl hydrogen orthophospphate (VOHPO.sub.4.1.5H.sub.2 O) having different crystal water such vanadyl hydrogen orthophosphate having different crystal water have different structures. Such fact is clear from the XRD pattern charts. The XRD patterns of the vanadyl hydrogen orthophosphate of the present invention and those of Datta et al are shown in FIGS. 6 to 9.
FIGS. 6 and 7 are XRD patterns of vanadyl hydrogen orthophosphate (VOHPO.sub.4.1.5H.sub.2 O) of the present invention. The vanadyl hydrogen orthophosphate having the XRD pattern shown in FIGS. 6 and 7 can easily be ion-exchanged with a divalent metal ion.
FIG. 8 is a XRD pattern of vanadyl hydrogen orthophosphate (VOHPO.sub.4.0.55H20) prepared by an organic medium in Datta et al and corresponds to FIG. 1-1 in Datta et al. FIG. 9 is a XRD pattern of vanadyl hydrogen orthophosphate (VOHPO.sub.4 0.5H.sub.2 O) prepared by an aqueous medium in Datta et al and corresponds to FIG. 1-2 in Datta et al.
Abdelouahab et al, Journal of Catalysis 157, 687-697 (1995) discloses a catalyst wherein VO(HPO.sub.4)0.5H.sub.2 O precursor of the vanadium phosphorus compound is doped with Fe and Co, but there are no descriptions what portion of the chemical structure of the phosphorus-vanadium compound is exchanged with Fe and Co. Therefore, it is unknown whether it is complexed oxide or mixture.
That is, we have noticed that the H.sup.30 present between the layers of vanadyl hydrogen orthophosphate hydrate, VOHPO.sub.4.nH.sub.2 O and by exchanging the H.sup.30 with a divalent metal ion, novel metal ion-exchanged phosphorus-vanadium compounds in the present application can be obtained. The resultant compound has an interlayer distance in the range of 7.0 to 8.2 .ANG., each of which is supported by the XRD (X-ray diffraction analysis) measuring results shown in Tables 1 and 2 in the present application. In particular, hkl=001 and the distance of C axis correspond to the interlayer distance.
In the Tables, the interlayer distance of ordinary vanadyl hydrogen orthophosphate hydrate, VOHPO.sub.4.0.5H.sub.2 O is 5.7 .ANG. and the interlayer distance of VOHPO.sub.4.1.5H.sub.2 O, the interlayer distance has been widen so as to be exchanged with a divalent ion, is 7.97 .ANG.(hkl=001, C axis). By ion exchanging VOHPO.sub.4.1.5H.sub.2 O with an ion, a new compound is obtained, the interlayer distance of which is 7.9 .ANG. for Ni, 7.84 .ANG. for Co, 7.31 .ANG. for Cu and 7.25 .ANG. for Zn, respectively.
However, Abdelouahab et al discloses only ordinary vanadyl hydrogen orthophosphate hydrate, VOHPO.sub.4.0.5H.sub.2 O. In according to the measuring results, on the catalyst being doped with Fe and Co, by XRD, it is clearly described that the doped catalyst has the same structure as that of ordinary vanadyl hydrogen orthophosphate hydrate, VOHPO.sub.4.0.5H.sub.2 O. (page 689, right column, line 43 onwards, in particular "only characteristic line of VOHPO.sub.4.0.5H.sub.2 O are observed.") In addition to the above, FIG. 2 shows that the face (001) is 5.7 .ANG. (2.theta.=15.50.degree.)
Consequently, the compounds of Abdelouahab et al are said to be mixtures of phosphorous-vanadium compound, Fe and Co or compounds in which Fe and Co are uniformly doped, so that the compounds of Abdelouahab et al are different from those of the present application.
No attempt has ever been made to synthesize a compound having a divalent metal cation exchanged for the H.sup.30 ion which intervenes between the opposed layers of the vanadyl hydrogen orthophosphate hydrate, a precursor of a phosphorus-vanadium oxide.
It is well known that the phosphorus-vanadium compound having such a plane structure as illustrated in the form of a model in FIG. 1, e.g. a vanadyl hydrogen orthophosphate hydrate, VOHPO.sub.4, is a layer compound which results from the hydrogen bondage through the medium of constitution water intervening between the VOHPO.sub.4, layers as shown in FIG. 2. The interlayer distance is generally 5.70 .ANG.. Absolutely no case of having this interlayer proton exchanged with other metal cation has been known to date.
An object of this invention, therefore, is to provide a novel phosphorus-vanadium compound.
Another object of this invention is to provide a novel phosphorus-vanadium compound which is obtained by the exchange of a proton present between opposed layers in the layer structure of a phosphorus-vanadium compound with a metal ion and a method for the production thereof.
Yet another object of this invention is to provide a novel solid acid catalyst.
A further object of this invention is to provide a catalyst appropriate for gas-phase partial oxidation of a hydrocarbon.